Cultured and propagated cerebrovascular endothelial cells form prostacyclin from endogenous arachidonic acid. Noradrenalin, isoproterenol serotonin but not histamine-increased prostacyclin (6-keto-PGF1alpha) synthesis from l00 to 200 percent. However the greatest increase of the synthesis (4.5-fold) is seen with the addition of calcium ionophore. The pattern of the lipoxygenase products obtained after the calcium ionophore stimulation of intact pieces of cerebral cortex was different from this seen in the endothelium. l2-L-hydroxyheptadecatrienoic acid (HHT) was found in significant amounts of brain tissue but not by endothelial cells indicating absence of thromboxane A2 formation in the latter.